– Propidium Iodide is a fluorescent intercalating agent that can be used to stain cells. Propidium Iodide is used as a DNA stain in flow cytometry to evaluate cell viability or DNA content in cell cycle analysis, and in microscopy to visualise the nucleus and other DNA-containing organelles. Propidium Iodide cannot cross the membrane of live cells, making it useful to differentiate necrotic, apoptotic and healthy cells.